1. Field of the Invention
The present invention pertains to the processing of signals received via an array antenna. In particular, the present invention pertains to methods and apparatus for increasing the effective resolving power of an array antenna.
2. Description of the Related Art
Array antennas are used in a wide variety of applications to transmit and receive directed beams of electromagnetic energy. An array antenna beam pattern, which typically includes a main lobe and side lobes, defines the angular dependence of the array gain. The shape and direction of an array antenna beam pattern are determined by the relative phases and amplitudes applied at the individual antenna elements that constitute the array via a process referred to as beamforming. For example, where hardware permits the relative phases of the antenna elements to be adjusted during operation, the main lobe of the antenna beam pattern can be steered over a range of different directions to transmit a signal in a selected direction or to receive a signal arriving from a particular direction.
When receiving a signal, received power is maximized by pointing the main lobe of the array antenna beam pattern in the direction of a source of a signal of interest. The resolving power of the array antenna is determined by the width of the beam pattern main lobe, commonly referred to as the array beamwidth. Assuming that the main lobe of the antenna beam pattern is pointed in the direction of a source of a signal of interest, a second signal from a source separated in angle by less than one array beamwidth may be identified as part of the signal of interest and may be amplified along with the signal of interest, thereby contributing significant interference to the signal of interest. The minimum beamwidth that can be achieved using beamforming techniques is determined, in part, by the number of array elements and the spacing between array elements. Hence, using conventional array signal reception techniques, the resolving power of an array antenna and the ability to receive a single isolated signal from among a plurality of signals from a plurality of closely spaced signal sources, is established by the physical characteristics of the array antenna.
Due to many of the same limitations identified above, conventional array signal reception techniques also limit the ability of an array antenna to identify and/or locate separate signal sources within a field of physical space. For example, a beam pattern's main lobe may be rotated, or scanned, through a field of physical space to determine the radial distribution of radiation sources relative a central boresight of the main lobe. Two radiation sources separated in angle by more than the beamwidth may be identified using conventional beamforming technique as two separate radiation sources. However, if two or more radiation sources are not separated in angle by more than one-half of the beam pattern beamwidth, the radiation sources will not be identified as separate radiation sources, but rather as a single source. As stated above, the minimum beamwidth that can be achieved using beamforming techniques is determined, in part, by the number of elements and the spacing between array elements. Hence, using conventional array signal reception techniques, the resolving power of an array antenna and the ability to identify separate signal sources within a field of physical space containing a plurality of signal sources, is established by the physical characteristics of the array antenna.
FIG. 1 depicts, graphically, a conventional approach to interference-rejection based upon the use of beamforming techniques. In an interference-rejection approach based upon beamforming techniques, a beam is formed on a signal of interest (FIG. 1 at 102) and a beam is also formed on the interference source (FIG. 1 at 104), typically by use of adaptive techniques. The contribution of the interference to the signal beam is then removed by subtracting the interference beam from the signal beam, resulting in an interference-rejection beam (FIG. 1 at 106) with a “null” for the angular direction of the interference. As demonstrated in FIG. 1 at 106, the results of such beamforming techniques typically produce a beam pattern that is distorted from the original beam pattern, at 102. For example, as shown in FIG. 1 at 106, the result of the subtraction, aside from being distorted, points in the wrong direction, yielding an erroneous direction for the signal of interest.
Conventional techniques based upon beamforming, as addressed above with respect to FIG. 1, are limited in their ability to reject interference from sources closely spaced in angle to a signal source of interest. Upon subtracting from the signal beam a beam formed on the interference source, a null is produced in the signal beam pattern; however, the nulling is not selective, as demonstrated in FIG. 1 at 106. The nulling will affect reception of a signal of interest as well as the interference. As signal and interference become closer together in angle the signal will, increasingly, also become rejected by the process along with the interference. This sets a limit on the minimum signal/interference separation that can be effectively dealt with in rejecting interference by means of beamforming techniques. Once again, the limitation is due primarily to the array beam pattern beamwidth, which, as explained above, is determined by physical characteristics of the array antenna. Hence, using conventional array beamforming techniques, the ability to reject as interference signals in close proximity to a signal of interest, is significantly limited by the physical characteristics of the array antenna.
Conventional signal processing approaches based upon beamforming techniques are limited in that the resolving power of the array is limited to the minimum beamwidth that can be achieved using the array. This sets a limit on the minimum signal/interference source separation that can be effectively dealt with in rejecting interference, isolating an individual signal and identifying individual sources of interference, as described above, as well as in other array signal processing applications. However, beamforming based techniques, as described above, may be effectively used in applications in which the signal/interference source separation is greater than one-half beamwidth. As a result, conventional beamforming/nulling techniques may be effectively used to remove any interference received form sources at angles corresponding to the beam pattern side lobes. For example, the beamforming/nulling techniques described above with respect to FIG. 1, may be used to remove interference incident upon an array antenna at angles corresponding to side lobes of the array beam pattern.
At least for the limitations identified above, a need remains for methods and apparatus for improving the effective resolving power of array antennas and for processing signals received via an array antenna that are not limited by the physical characteristics of the array antenna and the minimum beamwidth that can be achieved with the array antenna. Such techniques would preferably support improved signal isolation, signal source identification and interference-rejection without requiring changes to the physical characteristics of existing array antennas and/or other signal receiving hardware.